In general, important characteristics required to pencil leads are good writing feeling, a good color developing property of the drawn lines and high mechanical strength.
In order to provide a solid drawing material which is increased in an oil-impregnable effective pore volume and a surface area in a solid drawing material such as a pencil lead and further enhanced in compressive strength and has smooth writing feeling, a satisfactory color developing property and satisfactory drawn line intensity and which has a small abrasion amount and a good erasing property and is less liable to be stained when rubbing the drawn lines with a finger and a production process for the same, the present applicant proposes a solid drawing material prepared by forming a solid drawing material lead body by subjecting a blend composition for a solid drawing material containing at least a nanomaterial (nanoparticles) to burning treatment or non-burning treatment and filling pores of the above solid drawing material lead body with a lubricant (refer to, for example, patent document 1).
Further, it is known as well that a production process for a lead body prepared by adhering in advance a part or all of fine particles having an average particle diameter of 100 nm or less to a surface of a tabular extender to prepare a fine particle-adhered tabular extender, mixing and kneading it with lead body materials and then molding the mixture by extrusion molding is provided for the purpose of providing a process for producing a pencil lead which is an extrusion-molded lead body capable of forming a thick film containing fine particles in a high concentration by writing (refer to, for example, patent document 2). The above technique includes the content disclosed in patent document 1 described above. A thickness of the film described in the above patent document 2 is obtained by expressing merely power for masking the ground by a numerical value, and it is not related to the visual color (intensity) and the writing feeling.
On the other hand, the following defects are present in the evaluation item called “touch in writing” or “writing feeling” in patent document 1 described above. That is to evaluate it based on feeling obtained when a tester draws lines by a one-sidedly abraded face for a short time while maintaining a posture taken in starting the test without almost changing a writing face of the mechanical pencil. The one-sidedly abraded face is a face which is abraded and flat, and therefore the lines are written by a face which is almost abraded and flat until finishing writing since initiating it.
Accordingly, it has been found that when a lead to be tested is applied to a product developed by the present applicant which is put recently on the market and gains public favor (a mechanical pencil, trade name: “KURUTOGA”, manufactured by Mitsubishi Pencil Co., Ltd., international application WO2007/142135 (U.S. Pat. No. 4,240,417)), to be specific, when the test is carried out by applying it to a mechanical pencil of a type in which a lead is rotated whenever written to always carry out writing by a fresh part, the problem that such writing feeling that has so far been obtained is not reproduced is brought about.
That is, even if nanoparticles are simply mixed by the art described in patent document 1 and the like described above to form solid drawing materials, favorable evaluations of excellent drawn line intensity, writing feeling in actual writing and a static•dynamic frictional coefficient which is a typical index thereof have not been able to be obtained. When measuring a static•dynamic frictional coefficient for a solid drawing material obtained by simply mixing nanoparticles, the problem that the evaluation item called “the touch in writing” or “the writing feeling” described above is not always reproduced depending on a production process and a constitution of the lead body has been found.
As described above, the existing state is that pencil leads which have better and smoother writing feeling and high strength and which provide drawn lines with a deep and brilliant black color and a production process for the same are desired earnestly for pencil leads prepared by using nanomaterials (nanoparticles) and pencil leads used for a mechanical pencil of a type in which a lead is rotated whenever written to always carry out writing by a fresh part in addition to pencil leads used for conventional mechanical pencils, wood case pencils and the like.    Patent document 1: Japanese Patent Application Laid-Open No. 2007-138031 (claims, examples and others)    Patent document 2: Japanese Patent Application Laid-Open No. 2008-115211 (claims, examples and others)